1. Field of the Invention
This invention relates to rotodynamic pumps, and specifically relates to means for restricting fluid recirculation and for reducing wear between rotating and non-rotating elements of rotodynamic pumps, particularly those pumps suitable for handling slurries.
2. Description of Related Art
Rotodynamic pumps, such as centrifugal pumps, are commonly known and used for pumping fluids in many types of industries and for many applications. Such pumps generally comprise an impeller (rotating element) housed within a pump casing (non-rotating element) having a fluid inlet and fluid outlet, or discharge. The impeller is typically driven by a motor external to the casing. The impeller is positioned within the casing so that fluid entering the inlet of the casing is delivered to the center, or eye, of the impeller. Rotation of the impeller acts on the fluid primarily by the action of the impeller vanes which, combined with centrifugal force, move the fluid to the peripheral regions of the casing for discharge from the outlet.
The dynamic action of the vanes, combined with centrifugal forces resulting from impeller rotation, produce pressure gradients within the pump. An area of lower pressure is created nearer the eye of the impeller and an area of higher pressure results at the outer diameter of the impeller and in the volute portion of the casing. An area of pressure change from higher to lower exists in the radially extending gap between the rotating and non-rotating components. The pressure differential within the pump leads to fluid recirculation through the radial gap, between areas of high and low pressure. Such fluid recirculation, typically characterized as leakage, results in a consequent loss of pump performance and, in the presence of solid particles, a dramatic increase in wear. Therefore, pumps are structured with various sealing devices, both on the shaft side of the impeller to prevent external leakage and on the suction side of the impeller to prevent internal recirculating leakage.
Effective sealing arrangements are known and employed in pumps that process clear liquid. For example, U.S. Pat. No. 4,909,707 to Wauligman, et al., describes a floating casing ring that is positioned in the axially-extending radial gap between the impeller and the pump casing. Similar floating seal rings are described in U.S. Pat. No. 4,976,444 to Richards and U.S. Pat. No. 5,518,256 to Gaffal. U.S. Pat. No. 6,082,964 to Kuroiwa discloses a supported annular ring that is thereby allowed to float in surrounding fluid. Such sealing systems are directed to preventing leakage at the axially-extending radial gap between the rotating and non-rotating elements. These sealing arrangements may also include a wear ring element. One purpose of the wear ring is to reduce wear caused by contacting of the rigid components of the seal.
When pumps are used to process slurries, the abrasive particulate matter in the slurry causes wearing between rotating and non-rotating (i.e., stationary) elements of the pump. The wear dramatically increases when fluid recirculation occurs as previously described. Thus, an effective sealing means between rotating and stationary pump elements is desirable in order to effectively reduce fluid recirculation between the rotating and stationary elements of slurry pumps, and thereby effectively reduce wear.
Various examples of sealing arrangements for slurry pumps have been previously disclosed. Some sealing and/or wear ring arrangements have been disclosed for positioning in an essentially axially-extending radial gap between the impeller and the pump casing. Such sealing arrangements are disclosed in U.S. Pat. No. 3,881,840 to Bunjes and U.S. Pat. No. 5,984,629 to Brodersen, et al., both of which describe a fixed ring formed in the pump casing which interacts with a projecting element on the impeller to provide a labyrinthine seal and/or wear ring. It has to be noted that in general, axially-extending radial gaps are not well-suited for handling slurries due to high probability of solid particle entrapment between the rotating and non-rotating elements causing rapid wear in the pump elements.
Radially-extending axial gaps, or tapered gaps which are substantially radially-extending, are much less prone to entrapment of solids. Such sealing and leakage restricting arrangements are widely used in slurry pumps. U.S. Pat. Ser. No. 2004/0136825 to Addie, et al. discloses a fixed projection on either the pump casing or on the impeller to provide a leakage restricting arrangement between the impeller and the pump casing.
U.S. Pat. No. 6,739,829 to Addie discloses a floating ring element positioned between the impeller and pump casing which is also configured with means for receiving and distributing cooling and flushing fluid into the gap between the impeller and pump casing. Like other sealing arrangements, the floating ring seal of the '829 patent is purposefully sized and configured to provide a gap between the impeller and the sealing device to prevent friction between the seal and the impeller, and thereby prevent galling of the seal during rotation of the impeller. A necessary component of this design, therefore, is the presence of a flush system.
Prior sealing arrangements have heretofore been specifically directed to providing a seal that has sufficient clearance such that it does not contact the rotating elements of the pump, specifically to reduce or prevent wear and galling in the seal. As a result, such seal arrangements may still be vulnerable to undesirable fluid recirculation and wear between rotating and stationary elements of the pump. Moreover, placement of a sealing arrangement near the eye of the impeller in an axially-extending gap between the casing and impeller does not present the most effective means of preventing solid particle entrapment and subsequent wear between the casing and impeller.
Thus, it would be advantageous in the art to provide a relatively simple sealing arrangement which does not rely on a flush system and that effectively provides resistance to recirculation and wear between rotating and non-rotating elements of the pump, and one which is ideally located within the pump at a position where resistance to recirculation and wear can be most effective.